Namurian Stage

Namurian is the Stage name for rocks of Carboniferous age, which were formerly referred to as the Millstone
Grit Series. The Namurian rocks range in age from 326 to 315 million years*

Distribution

Rocks of Namurian age crop out along the western side of the Sheffield region and the adjacent Dark Peak area
of the Peak District National Park. The Namurian sequence, which is at least 600 m thick, consists of
alternating sandstones, gritstones, mudstones and shales, forming the eastern limb of the broad Pennine
anticline. Differential erosion of the easterly dipping strata has resulted in a succession of spectacular
west-facing escarpments, known locally as "edges" e.g. Stanage Edge, beloved by climbers.

Namurian rocks throughout the sequence have long featured in the economy of the region, e.g. as building
stones, as grinding stones, as raw materials for the cement industry and with the potential for hydrocarbon
resources. The deep valleys, floored by impermeable shales, have provided ideal sites for water reservoirs,
to supply Sheffield and other cities in the Midlands. Only the younger Namurian strata crop out in our area,
but they are underlain by the full succession, so a brief description of each major unit will be given,
whether it is exposed at the surface in our district or in the Peak District to the west of this area
(Rocks of Namurian age are shown in green on the geological map of our region - see "South
Yorkshire Geology" map.)

Tectonic setting

The Namurian is preceded by prolonged deposition of predominantly carbonate sediments during the Visean,
which took place in a number of marine basin and platform structures, lying within the overall Pennine
Basin. The upland area known as the Wales-Brabant High lay to the south, and the Caledonian mountains
of Scotland/Laurentia/Scandinavia lay further away to the north. This broad pattern continued
throughout the Namurian and into the overlying Westphalian. (See "cartoon" of the global
setting for these times under "Westphalian" Figure 1. See Figure 2, below, for the broad regional
palaeogeography for much of the Namurian)

Sedimentation was dominantly of clastic material (i.e. gravel, sand, silt and mud), derived from the upland areas, in contrast to the carbonates
of the Visean below. This is thought to be due to renewed uplift of the source areas in the ancient continental
area to the north, perhaps accompanied by increased rainfall and run-off from the land, compared to more
arid conditions and lower surrounding land in the Visean. Early Namurian deposition consisted mainly of deep
water marine muds, which accumulated very slowly, to judge from the numerous bands of evolving goniatites.
Later sedimentation reflected the gradual advance of a major river delta from a broadly northerly source,
bringing at first distal muds and silts, to be followed by turbidites and coarsening-upwards sequences,
culminating in the coarse grits of successive delta tops. Later, successive uplift/subsidence events
resulted in the return of deep water conditions and the deposition of shales. As the sedimentation continued
and water depth decreased, sands were again deposited in delta systems. Some of these briefly even became land
areas, marked by localised erosion and the deposition of thin coal seams.

Stratigraphy

The Namurian rocks of the South Pennines are divided into four regional sub-stages (divisions of time), based on the fossils they
contain. From youngest to oldest these are Yeadonian, Marsdenian, Kinderscoutian and Pendleian-Alportian,
all named after places in the South Pennines. For field purposes, the traditional division based on the lithology
of the rocks is probably more practical. On this basis, the Namurian is divided into two major Groups. The younger is
the Millstone Grit Group, which includes the upper three sub-stages and consists
of coarse-grained sandstones with angular grains, known as grits, although there are also substantial
thicknesses of finer-grained sandstones, siltstones, shales and occasional thin coals.The older
of these is the Craven Group, which is dominated by fine grained sediments, collectively known as the Bowland
Shale Formation of Pendleian-Alportian age.

The evolution of the Pennine Delta is summarised in the "cartoon" below (Fig. 10). The
underlying Visean limestones are shown in the bottom right, followed successively by the marine muds of the
Edale Shales; the distal turbidites of the Mam Tor Beds; the proximal turbidites and subaqueous channels
of the Shale Grit; and the delta-top sediments of the Kinderscout Grit.

Fig 10: Diagram of the delta margin and slope deposits in the Edale-Mam Tor area
from Ford, T.D. 2002. Rocks and Scenery of the Peak District, p29, Landmark Publishing